1. Field of the Invention
The present invention relates generally to a light receiving circuit for use in a tuning pulse generator of an electronic tuning type receiver, and more particularly, to a light receiving circuit having a high response speed and minimal output fluctuation for use in a tuning pulse generator of an electronic tuning type receiver.
2. Description of the Prior Art
As a result of rapid progress in electronic technology, receivers have been extensively improved. Recently, an electronic tuning system has been employed in a receiver. Such an electronic tuning system includes a pulse generator for generating up-pulses or down-pulses in accordance with manual operation by the user. The pulse output from the pulse generator is counted by an up-down counter, which provides a digital bit stream output. The output digital bit stream is then converted to an analog signal in the form of a station selection voltage, which is applied to a variable capacitance diode (varactor diode) in the tuning section thereby to achieve the desired station selection.
In this pulse generator, a disk having a number of slits is rotated in association with a station selecting knob so as to be operated similarly to the conventional variable tuning capacitor. Light passed through a slit in the disk is received by two light receiving elements, e.g., photo-diodes, which are positioned in the rotation direction of the disk slightly apart from each other. From the relationship between the outputs of the two light receiving elements, up-pulses, down-pulses and clock pulses are obtained.
The conventional light receiving circuit employed for each of the two light receiving elements is shown in FIG. 1.
Referring to FIG. 1, the emitter of a photo-transistor 1 is grounded, and the collector thereof is connected through a resistor 2 to a power supply +B. A transistor 3 receives as its base input the voltage present at the connection point P of the photo-transistor 1 and the resistor 2. The emitter of transistor 3 is grounded, and the collector thereof is connected through a resistor 4 to the power supply +B. An output terminal 5 is connected to the collector of the transistor 3.
When the photo-transistor 1 receives light, the resistance value of the photo-transistor 1 is varied in accordance with the intensity of the light, as a result of which the voltage level at connection point P is correspondingly varied. The voltage at connection point P is applied to the base of the transistor 3, and is amplified by transistor 3. The amplified light receiving signal appears on the output terminal 5.
However, the conventional light receiving circuit described-above has deficiencies in that it has a slow response speed to the input light and its high or low level outputs fluctuate greatly.